1. Field of the Invention
The present invention generally relates to a multihop relay broadband wireless communication system, and in particular, to a method for relaying a signal by selecting a relay mode depending on a channel status of links between a Base Station, a Relay Station, and a Mobile Station in a multihop relay broadband wireless communication system, and a Relay Station apparatus for supporting the method.
2. Description of the Related Art
In fourth-generation (4G) mobile communication systems, cells having a very small radius are located to enable rapid communications and accommodate more traffic. However, it may be impossible to achieve a centralized design using current wireless network design schemes. Wireless networks should be controlled and deployed in a distributed manner, and actively adapt to environment changes, such as a joining of a new Base Station. To these ends, 4G mobile communication systems should be configured as autonomous adaptive wireless networks.
Techniques applied to an ad-hoc network are typically adopted by a mobile communication system for substantial implementation of an autonomous adaptive wireless network by a 4G mobile communication system. A representative example is a multihop relay broadband wireless communication system, in which a multihop relay scheme applied to an ad-hoc network is introduced to a broadband wireless communication system configured with a fixed Base Station. In the broadband wireless communication system, since communications are conducted through one direct link between a Base Station and a Mobile Station, it is easy to establish a highly reliable radio communication link between the Base Station and the Mobile Station.
However, since the wireless network configuration of the broadband wireless communication system has low flexibility because of the fixed Base Station, it is hard to provide efficient services in a radio environment, which is subject to severe change in traffic distribution or traffic. To overcome this shortcoming, it is possible to apply a relay scheme which delivers data in a multihop manner by use of neighboring Mobile Stations or Relay Stations. A multihop relay scheme can rapidly reconfigure the network under the environment change. Also, a multihop relay scheme can provide a Mobile Station with a radio channel of better channel status by building a multihop relay path by way of a repeater which is placed between the Base Station and the Mobile Station. Furthermore, a high speed data channel can be provided to Mobile Stations which cannot communicate with the Base Station in a shadow area, by means of the multihop relay path, to thereby expand the cell area.
FIG. 1 depicts a multilink configuration of a general multihop relay broadband wireless communication system.
As shown in FIG. 1, a Mobile Station (MS) 110 in a coverage 101 of a Base Station (BS) 100 is connected to BS 100 through a direct link. In contrast, a MS 120 with poor channel status, which resides outside the coverage 101 of BS 100, is connected to a relay link via a Relay Station (RS) 130.
When MSs 110 and 120 suffer poor channel status because they are outside the coverage 101 of BS 100 or in a shadow area under the severe shielding by buildings, BS 100 is able to provide better radio channels to MSs 110 and 120 by means of RS 130. Accordingly, by adopting the multihop relay scheme, BS 100 can provide a high speed data channel in the boundary area of the poor channel status and expand the cell service area. To transmit uplink (UL) and downlink (DL) communications between the BS 100, the RS 130, and the second MS 120, a BS-RS link between the BS 100 and the RS 130, an RS-MS link between the RS 130 and the second MS 120, and a BS-MS link between the BS 100 and the first MS 110 are established. Each link is divided to the UL or the DL according to the data transmission path. The respective links (BS-RS link, RS-MS link, and BS-MS link) are established independently from one another.
To relay signals between the BS and the MS, the RS adopts an Amplify and Forward (AF) scheme or a Decode and Forward (DF) relay scheme. The AF scheme and the DF scheme work using different Open System Interconnection (OSI) layers, as shown in FIG. 2.
FIG. 2 depicts the OSI layer structure of a general RS for performing relay modes. The OSI layers employ the OSI layers of Institute of Electrical and Electronics Engineers (IEEE) 802.11a.
As shown in FIG. 2, being processed in a Physical Medium Dependent (PMD) sublayer and a Packet Level Control Protocol (PLCP) sublayer of a physical layer, the AF scheme merely amplifies and forwards the received signal.
Being processed in the Medium Access Control (MAC) layer as well as the physical layer, the DF scheme decodes and forwards the received signal after encoding and modulating the received frame. Advantageously, the DF scheme can obtain an additional coding gain by encoding the received signal differently from the coding scheme of the received signal depending on the channel status of the BS-RS link and the RS-MS link.
As discussed above, in the multihop relay broadband wireless communication system, the respective links (BS-RS link, RS-MS link, and BS-MS link) are established independently from one another. The RS relays the signal using a preset relay scheme (e.g., AF scheme or DF scheme).
Using an AF relay scheme, when the channel status of the BS-RS link and the RS-MS link are good, the RS can relay the signal. However, under a poor channel status of the RS-MS link, the RS merely amplifies the signal of the same modulation and coding as in the BS-RS link of the good channel status, and forwards it. As a result, the MS is not able to detect the relayed signal.
In the event of a poor channel status of the BS-RS link, the RS merely amplifies and forwards the signal distorted in the BS-RS link to the MS, to thus amplify the noise as well.